The role of proton-bridged intermediates in promoting hydrogen-deuterium exchange in gas-phase protonated diamines, peptides and proteins

The role of proton-bridged intermediates in gas-phase hydrogen-deuterium exchange is illustrated with a series of diamines, peptides and proteins. Proton-bridged intermediates account for the relatively high reactivity of protonated and multiply protonated species when the deuterating reagent (such as CH3OD) is significantly less basic than the conjugate base of the protonated species. The absence of these intermediate greatly diminishes the rate of exchange. The rate constants are determined as apparent (k(apparent)) and site specific (k(site)) for the diamines. The effects of ring size, stereochemistry and proton affinity are explored. The presence of proton-bridged intermediates in peptides and proteins is facilitated by the presence of lysine residues. Lysine residues, when found in high proportion, increase the rates of hydrogen-deuterium exchange for the peptide or protein. (C) 1998 Published by Elsevier Science B.V. All rights reserved.